Surface irrigation of pasture with treated sewage effluent

Abstract Surface irrigation of pasture with treated sewage effluent at Templeton, Christchurch, for up to 16 years, had little effect on the heavy metal composition of the soil or pasture. Slightly higher levels of 0.1n HC1‐extractable Zn, Cu, Co, and Mn in the effluent‐treated soil did not increase levels of these elements in the pasture, and did not increase the Cd/Zn ratio. The absence of any marked changes in heavy metal content is attributed to the low levels of these elements in the raw sewage effluent, which is of domestic origin. It is concluded that at the present effluent application rate (840 mm/year) the scheme, without sludge disposal, has an essentially indefinite life span, in terms of heavy metal accumulation. If land application of the sludge is introduced, it would take at least 200 years for heavy metals to reach the recommended limit of 5% of the soil cation exchange capacity.